This invention relates to charge balancing electronic circuitry. More specifically, this invention relates to low cost, linear charge balancing circuits that automatically balance voltages between two or more charge storage devices, such as capacitors, connected in series.
Charge storage devices, such as capacitors and ultracapacitors configured to provide a short term power supply or boost, are often used as part of a device bank comprising a series string of devices. The use of a bank of capacitors, rather than a single capacitor, can provide higher voltage delivery or a greater capacity through increased amp-hours. Individual capacitors included in such a bank, however, can have a tendency to accept a charge at different rates or can become unbalanced through charging or discharge. This imbalance can cause overvoltage problems causing one or more of the capacitors in the bank to have a catastrophic failure.
In order to address these problems, a number of complicated solutions have been proposed which protect against a capacitor becoming overcharged to the point of failure. For example, one such solution comprises using diode-type devices, such as zener diodes, to shunt each capacitor in the bank when the individual capacitor reaches a predetermined threshold. The value of each diode-type device must be specifically selected for each particular application to match the predetermined threshold for that application. Once the capacitor is charged to the predetermined threshold, the diode-type device causes any continuing charging current to be shunted around the capacitor protecting the capacitor from overcharging.
The diode-type device solution, while capable of providing overcharging protection, is imperfect in that it comes with various disadvantages in its implementation. For example, usable diode-type devices typically do not provide a perfect shunt around the capacitor. As such, the devices dissipate energy, typically in the form of heat. This dissipated energy is wasted energy in that it is not being used to charge capacitors. The heat created during energy dissipation can also cause overheating problems in certain applications. Another disadvantage of diode-type devices is that they continue to draw current even after all capacitors in the bank have been charged. This continual current draw leads to additional wasted energy. One additional disadvantage of diode-type devices is that the diode values must be carefully selected for each specific application. In other words, each diode-based system is customized to a particular application. This inflexibility presents design and manufacture problems in that each new application requires a redesign of the diode values. Thus, it is difficult, and indeed in many cases impossible, to build a standard diode-based charge balancing circuit that can be used with various different capacitor banks.
Another proposed solution for a charge balancing circuit includes a passive resistive bridge comprising a series of resistors arranged in a predetermined configuration. Like the diode-type devices described above, resistive bridges also suffer from various disadvantages in their implementation. For example, resistive bridges typically leak substantial amounts of energy and, like diode-type devices, continue to leak energy even after the capacitors in the bank are fully charged. In addition, resistive bridges substantially slow down the time required to charge a capacitor bank.
More recently, solutions for charge balancing have been proposed which comprise complicated microprocessor-driven circuits that monitor such things as the charge and/or the charge/discharge rate of each individual capacitor in a bank as well as the overall charge of the entire bank. These circuits typically include switching logic, inductors, and/or other components that can be controlled by the microprocessor to protect each individual capacitor from becoming over charged. One major disadvantage of such microprocessor-driven circuits is that they are typically complex and expensive devices. While the microprocessor-driven circuits provide monitoring, recording and tracking capabilities which are typically not found in the solutions mentioned above, these additional capabilities are often duplicative of capabilities already available in the end user application in which the capacitor bank is applied. Thus, the added expense and complexity associated with microprocessor driven charge balancing circuits are often unnecessarily wasteful. Another disadvantage is that microprocessor-driven solutions typically come with a high quiescent current. Thus, to minimize energy waste caused by the high quiescent current, the microprocessor must be capable of being turned off when it is not needed. The control logic needed for turning the microprocessor on and off at the appropriate time further increases the expense and complexity of these microprocessor-driven circuits.
Thus there is a need for a simple, inexpensive, flexible charge balancing circuit that minimizes energy waste while providing overvoltage protection for individual capacitors in a capacitor bank.
These and other needs are satisfied by a charge balancing circuit according to the present invention which comprises a voltage divider, an operational amplifier, and a negative feedback resistor. In one embodiment, a charge balancing circuit according to the present invention can be configured for balancing the charge for two series connected charge storage devices such as capacitors. In addition, charge balancing circuits can be xe2x80x9cstackedxe2x80x9d to provide charge balancing and overvoltage protection for a bank of any length series string of capacitors.
The voltage divider is configured to equally divide the charge voltage across the capacitors in the bank and provide an input to the operational amplifier. The negative feedback resistor is configured to provide feedback information to another input of the operational amplifier, the feedback information relating to the voltage of the capacitors. In this manner, if the voltage of one of the capacitors is higher than the voltage of the other capacitor, the inputs to the operational amplifier are unbalanced. The operational amplifier is configured to provide an output current when the voltage divider input and feedback input do not match thus causing energy from the capacitor having a higher voltage to be transferred to the capacitor having a lower voltage.
In one embodiment, the voltage divider comprises two divider resistors connected to an input of the operational amplifier. Preferably, the divider resistors are of approximately the same value and the value of the resistors is high enough to minimize the quiescent current of the circuit. Also, preferably, the value of the negative feedback resistor can be approximately half that of the divider resistors so that the negative feedback resistor can cancel any input bias current supplied to the operational amplifier.
A current limiting resistor can also be included between the operational amplifier output and the capacitors. The current limiting resistor can be configured to limit the output current of the operational amplifier to a safe level. The voltage drop across the current limiting resistor can also provide information regarding the health of the capacitors being balanced by the charge balancing circuit. For example, the voltage drop across each current limiting resistor can be compared to the average voltage drop of all of the current limiting resistors. If the voltage drop of any current limiting resistor is significantly higher than the average, a problem may exist with one of the capacitors being serviced by the current limiting resistor.
In another embodiment, a gain stage can be included for increasing the output current of the operational amplifier. The gain stage can be particularly useful for banks employing large capacitors. The increased output current produced by the gain stage can be sufficient to overcome the leakage current of larger capacitors and can also increase the speed in which the capacitors are balanced. Preferably, the gain stage comprises two transistors connected between the operational amplifier output and the capacitors.